1. Technical Field
The present application relates to a method to control a gas engine and a gas engine system thereof, the engine being provided with: either a turbocharger or a supercharger through which air is supplied to the engine [henceforth in this application, whenever the word ‘turbocharger’ appears, it could equally well be replaced by ‘supercharger’]; a first gas control valve that controls flow-rates of fuel-gas to be supplied to each cylinder of the engine; whereby, the fuel-gas that is regulated by the first gas valve and the air that is supplied through the turbocharger are mixed so as to form a prescribed air-fuel ratio; and, the engine burns the supplied fuel-gas under conditions of the prescribed air-fuel ratio; specifically, the engine can be operated based on the air-fuel-ratio control with high precision, even in the case when a low calorific fuel gas that is prone to vary in calorific value is used.
2. Description of the Related Art
A conventional small gas engine, such as an engine with a cylinder bore of approximately 200 mm or less, usually adopts a fuel-gas mixing system in which fuel-gas and air are mixed upstream of air-inflow before a turbocharger, while the fuel-gas air mixture is supplied to the main combustion chambers i.e. cylinders, through the turbocharger (i.e., a compressor thereof) and an air cooler.
On the other hand, in conventional larger gas engines, fuel-gas is supplied into each cylinder, through a charging air inlet branch arm pipe just before each cylinder and through a gas supply control valve for each cylinder, as fuel-gas supply is required to be uniform all over the cylinders in amount as well as in gas concentration distribution, as a rule. In this manner, not only the air fuel ratios and the fuel quantities over the cylinders can be equalized but also the fuel-gas charging is streamlined; further, since fuel-gas and air are mixed just in front of each cylinder, the length of potentially flammable gas-air-mixture flow that is formed upstream of each cylinder can be shortened so as to enhance engine operational safety against explosion risks.
Hereby, it is noted that the above-mentioned gas supply control valve for each cylinder is also called a first gas valve in this specification; and, the first gas valve is often termed a gas admission valve in the larger gas engine field, because it is the valve which is provided at each cylinder of the gas engine in principle; on the other hand, as described later in this specification, a term, namely, a second gas valve, is introduced for a fuel-gas supply control valve that supplies fuel-gas to suction air. Further, a fuel-gas supply line that is connected to the first gas valve is called a first gas line in this specification; in the same way, a second gas line is defined in response to the second gas valve.
A patent reference 1 (JP2001-132550) discloses a technology in which fuel-gas supply methods of the mentioned smaller and larger gas-engines are combined. In a gas engine according to the reference 1, fuel-gas pressurized by means of a gas compressor is supplied to a cylinder inlet of a charging air passage or a cylinder, whereas fuel-gas that is not compressed by the compressor is supplied to the upstream side of air-inflow line before a turbocharger, from a gas inflow line (a gas source) before the gas compressor; further, the fuel-gas supply can be switched from the line toward the cylinder inlet, to the line toward the upstream side of the air-inflow line before the turbocharger and vice versa.
In the larger gas engine in which fuel-gas is supplied to a cylinder inlet of a charging air passage or a cylinder, it is required that the fuel-gas pressure at the inlet of the cylinder be higher than the supercharged air pressure. As a result, in the case when a low calorific fuel gas i.e. a gas of a low calorific value such as coal mine methane-gas, a gas compressor of a large capacity is needed so as to compress a fuel-gas of low pressure (substantially less than an ambient pressure) and large flow rates.
On the other hand, in a gas engine with a fuel supply system in which fuel-gas is supplied to the upstream side of the air in-flow line before the turbocharger, a flammable air fuel-gas mixture is compressed into a state of high temperature and a high pressure in response to a substantially adiabatic compression process through the turbocharger compressor; hence, potential risks of gas explosion are involved so long as the mentioned fuel supply system is employed.
Against the above backdrop, a patent reference 2 (JP 2006-249954) discloses a technology as to a gas supply device and an operation method thereof for the aforementioned larger gas engines. In the technology according to the reference 2, the fuel-gas device comprises:
a second fuel-gas supply line, i.e. a second gas line, through which a part of fuel-gas is mixed with air that is inducted by the turbocharger compressor;
a first fuel-gas supply line, i.e. a first gas line, through which the remaining part or the whole part of fuel-gas is mixed with air or air-fuel mixture at a gas supply branch arm pipe upstream of each cylinder;
a gas supply control valve, i.e. a second gas valve, for suction air, the valve regulating fuel-gas amount (flow rates) to be supplied through the turbocharger compressor;
a gas supply control valve, i.e. a first gas valve, for each cylinder, the valve regulating fuel-gas amount (flow rates) to be supplied through the gas supply branch arm pipe upstream of each cylinder;
a gas (fuel-gas) compressor that is provided at the upstream side of the first fuel-gas supply line;
whereby the amount (flow rates) of the fuel-gas through the second gas line is controlled by means of regulating opening levels of the second gas valve for suction air, so that the concentration of the supplied fuel-gas in the air fuel-gas mixture that flows through the turbocharger compressor is kept below the lower (lean) limit of flammability as to the fuel-gas.
According to the above disclosure, potential risks of fuel-gas explosion that may occur in the neighborhood of the turbocharger compressor outlet are completely eliminated; further, the size and capacity of the gas (fuel-gas) compressor that compresses fuel-gas and supplies the fuel-gas to the gas supply branch arm pipe upstream of each cylinder can be reduced because of reduced power consumption, even in the case when a fuel gas with a low calorific value is used.
Hereby, it should be noted in addition that the patent reference 2 utilizes a technology originated from larger gas engines that are provided with first gas valves in principle, whereas small gas engines are not provided with a first gas valve fitted to each cylinder in general; namely, a small gas engine includes a gas valve that supplies fuel-gas toward the air suction line of the engine. The gas valve that supplies fuel-gas toward the air suction line is termed a second gas valve in this specification.
Further, it is also noted that the entire contents of the patent reference 2 (Application No. JP 2006-249954 filed on Mar. 8, 2005 with Japanese Patent Office) are hereby incorporated by reference.
As described above, according to the reference 2, a gas engine can be realized, whereby a sufficient amount (flow rates) of fuel-gas can be secured and in addition, the fuel-gas compressor can be of a smaller size and capacity. However, a further advanced technology has been anticipated, whereby fuel-gas amount (flow rates) through the second gas line toward the turbocharger compressor can be precisely controlled with a simple mechanism, and the further advanced technology can provide a control method to be applied even to the case where the calorific value of the fuel-gas continuously varies.